Conventionally, a construction machine including a boom as the driven body, a boom cylinder that rotates and drives the boom, a hydraulic pump that supplies hydraulic oil to the boom cylinder, and a control valve that controls the supply of the hydraulic oil to the boom cylinder and the discharge of hydraulic oil from the boom cylinder is known.
In the construction machine, a lock valve for locking the boom so as not to rotate in the lowering direction due to its own weight when the work of the construction machine with the boom raised is suspended (when the control valve is operated to a neutral position) is provided.
The lock valve is provided between the control valve and the boom cylinder in order to prevent leakage of the hydraulic oil in the control valve.
As in a construction machine disclosed in Japanese Unexamined Patent Application Publication No. 2008-274988 illustrated in FIG. 8, a plurality of control valves may be connected to a boom cylinder.
Specifically, the construction machine includes first and second hydraulic pumps 101A and 101B that supply hydraulic oil to a boom cylinder 100 and a valve unit 102 that controls the supply of the hydraulic oil to the boom cylinder 100 and the discharge of the hydraulic oil from the boom cylinder 100.
The valve unit 102 includes a first control valve 103A connected to the first hydraulic pump 101A, a second control valve 103B connected to the second hydraulic pump 101B, and a valve body 104 that stores both control valves 103A and 103B and has passages R100 to R103 described later.
The first control valve 103A is connected to the first hydraulic pump 101A through a pump passage R100 and the second control valve 103B is connected to the second hydraulic pump 101B through a pump passage R103.
Moreover, both control valves 103A and 103B are connected to a head-side chamber of the boom cylinder 100 through a head-side passage R101 and a rod-side chamber of the boom cylinder 100 through a rod-side passage R102.
For example, when both control valves 103A and 103B are switched to a boom raising position, the hydraulic oil output from both hydraulic pumps 101A and 101B through both control valves 103A and 103B converges in the head-side passage R101 and is guided to the head-side chamber of the boom cylinder 100.
Here, since the head-side passage R101 and the rod-side passage R102 are formed inside the valve body 104, the cross-sectional area of both passages R101 and R102 is limited. As a result, there is a problem in that the pressure loss in the hydraulic oil increases in the converging portion of the head-side passage R101 and the rod-side passage R102.
Thus, in order to suppress the pressure loss, parallel passages respectively connected to both control valves 103A and 103B may be formed in the valve body 104 and these passages and the boom cylinder 100 may be connected by a converging hydraulic pipeline (external hydraulic pipeline).
In such a configuration, when the lock valve described above is employed, the lock valve is connected between the valve body 104 and the converging hydraulic pipeline. That is, the lock valve is connected to each of the two control valves 103A and 103B.
The lock valve includes a valve element capable of moving between a locking position at which the discharge of the hydraulic oil from the boom cylinder is restricted and an unlocking position at which the discharge of the hydraulic oil from the boom cylinder is allowed. The valve element is disposed at the locking position in a work suspended state and moves to the unlocking position before the boom cylinder is driven.
However, when the valve element moves from the locking position to the unlocking position, a space in which the hydraulic oil can flow is formed in the passage of the hydraulic oil by movement of the valve element. Due to this, when the hydraulic oil flows into this space, the rod of the boom cylinder moves and a shock resulting from this movement occurs.
In particular, as described above, when a plurality of (two) lock valves is provided with respect to a boom cylinder and these valve elements move to the unlocking position at the same time, the shock may increase and an operator may experience unpleasant feeling.